1. Technical Field of the Invention
The present invention relates to a power circuit, for example, capable of preventing efficiency from being reduced by an increase in switching loss of a DC/DC converter when a wideband envelope signal is received.
2. Description of the Related Art
According to the related art, when a radio frequency signal such as a CDMA (Code Division Multiple Access) signal or a multi-carrier signal is power-amplified, a distortion compensation function is added to a common amplifier and an operation range of the common amplifier is expanded up to the vicinity of a saturation region, so that low power consumption is achieved. The distortion compensation function includes a feed-forward compensation function, a pre-distortion compensation function or the like, but there is a limitation in the lowering of power consumption by only using the distortion compensation. In this regard, in recent years, a method of achieving high efficiency by using a saturated amplifier has been spotlighted. Particularly, it has been considered that a scheme for changing the power of the saturated amplifier is effective.
Further, in terms of demands for a power amplifier of a transmitter, in order to reduce the limitation on installation location or installation cost, reduction in size and weight thereof has been strongly required. The volume or weight of an apparatus increases with the number of fins for dissipating heat generated by power loss. However, the number of the fins is reduced by an improvement in power efficiency, resulting in reduction in size and weight of the apparatus.
A method of improving the power efficiency includes an EER (Envelope Elimination and Restoration) scheme of changing a supply voltage of a saturated power amplifier.
FIG. 5 is a block diagram showing a configuration example of an EER scheme of changing power by using a saturated amplifier according to the related art.
According to the configuration example, a distributor 111, an envelope detector 112 and a power circuit 113 provided on one distribution path, and a RF (Radio Frequency) limit amplifier 114 and a main amplifier 115 provided on the other distribution path, are provided between an input terminal 101 and an output terminal 102.
An RF signal input from the input terminal 101 is distributed by the distributor 111.
In relation to a part of the distributed signal, an evelope is detected by the envelope detector 112, and power output of the power circuit 113 is changed according to a signal (amplitude information) of the detected envelope. Thus, the power circuit 113 changes a supply voltage of the main amplifier 115 according to the input envelope signal.
In relation to the other part of the distributed RF signal, amplitude variation is removed by the RF limit amplifier 114 and the RF signal is amplified in a saturated state by the main amplifier 115 while only information on a phase component is being maintained.
Herein, since power (power from the power circuit 113) of the main amplifier 115 is changed according to the amplitude information, the amplitude information is restored. Since the amplifier (the main amplifier 115) is always used (operates) in a saturated state, efficiency thereof is good. Thus, high efficiency is achieved.
Since the main amplifier 115 operates in the saturated state, high efficiency is achieved. However, in terms of the entire efficiency of the EER scheme, the efficiency of the power circuit 113 is also important. For example, a band of the envelope signal of a wideband signal such as a W (Wideband)-CDMA signal or an OFDM (Orthogonal Frequnecy Division Multiplexing) signal is wide, and the power circuit 113 has to operate at a high speed.
For example, a band of wideband envelope information such as a CDMA signal or a multi-carrier signal is wide, and an envelope amplifier as shown in FIG. 6, which changes power, has been known as a power circuit operating at a high speed (For example, refer to “An Improved Power-Added Efficiency 19-dBm Hybrid Envelope Elimination and Restoratin Power Amplifier for 802.11g WLAN Applications”, IEEE MTT, VOL. 54, NO. 12, 2006).
FIG. 6 is a circuit diagram showing a configuration example of an envelope amplifier (power circuit) that changes power.
In such a method, switching power, which is assisted by a linear amplifier employed in an audio amp or the like, is used. In general, it is referred to as a linear assist class BD amplifier (For example, refer to “A Class B Switch-Mode Assisted Linear Amplifier” IEEE PE, VOL. 18, NO. 6, 2003, and “Series- or Parallel-Connected Composite Amplifiers” IEEE PE, NO. 1, 1986).
The envelope amplifier according to the example includes an OP amp 12, a hysteresis comparator 13, a current detector 14 and a DC (Direct Current)/DC converter 15, which are provided between an input terminal 1 and an output terminal 2.
The DC/DC converter 15 includes a supply voltage 31, a switch device 32, a diode 33 and an inductance 34.
Further, FIG. 6 shows nodes P, P1 and P2.
As described above, the circuit includes the OP amp 12 serving as a wideband voltage source, the DC/DC converter 15 with high efficiency, the hysteresis comparator 13 serving as a control circuit, and the current detector 14.
The circuit operates in a follow mode and a non-follow mode.
First, the follow mode will be described.
A signal detected by the envelope detector 112 as shown in FIG. 5 is input to the input terminal 1, and converted into a voltage source by the OP amp 12. When output from the envelope detector 112 is a DC portion, a voltage of the node P1 of the current detector 14 is increased, and the hysteresis comparator 13 operates to turn on the switch device 32. The supply voltage 31 is applied to the node P, which serves as a connection point of the switch device 32 and the inductance 34, and passes through the inductance 34 so that a voltage of the output terminal 2 is gradually increased.
If the output of the output terminal 2 is higher than the output from the OP amp 12, the node P2 becomes high and the hysteresis comparator 13 turns off the switch device 32. Thus, current flowing through the inductance 34 flows through the diode 33, and the output of the output terminal 2 is gradually reduced. Then, the hysteresis comparator 13 turns on the switch device 32, and repeats the above operation. That is, the hysteresis comparator 13 self-oscillates to control the switch device 32.
The self-oscillating frequency is determined by a hysteresis width with the degree of freedom, the inductance 34, the supply voltage 31, and a resistance value of the current detector 14. However, if the self-oscillating frequency is set to be high, since switching loss is increased or it exceeds a limit value of the switch device 32, the self-oscillating frequency is limited.
Further, when the output from the envelope detector 112 has DC and AC components and it is a low frequency portion, similar to the case in which the output from the envelope detector 112 is the DC component, PWM (Pulse Width Modulation) of the DC/DC converter 15 is made to follow, so that output power is supplied from the DC/DC converter 15 with high efficiency.
Second, the non-follow mode will be described.
If the output from the envelope detector 112 is the DC and AC components of a high frequency, the PWM of the DC/DC converter 15 is not made to follow, so that the output power is supplied from the OP amp 12. That is, if the output from the envelope detector 112 is the DC and the AC with the high frequency, the DC component and AC component of a low frequency are supplied from the DC/DC converter 15 because the AC component of the high frequency is removed by the inductance 34 from the output of the DC/DC converter 15. The AC component of the high frequency is supplied from the OP amp 12.
At this time, DC current and AC high frequency component are generated at the nodes P1 and P2 of both ends of the current detector 14, so that the output from the hysteresis comparator 13 operates the switch device 32 by using the high frequency of the AC component employed as a basic frequency.
According to a method of improving the efficiency of the power circuit, for example, the AC component, which can be made to follow due to an increase in the self-oscillating frequency, can be increased up to a high frequency (high frequency portion). However, since a band of the communication system such as the WiMAX or the LTE is wide and a band of the envelope signal becomes wider, the AC component is limited.
Thus, in the case of the follow mode, the voltage is supplied from the DC/DC converter 15 to the output terminal 2, so that efficiency is improved. Further, in the case of the non-follow mode, the AC component of the high frequency is supplied from the OP amp 12, and the AC component of the low frequency and the DC component are supplied from the OP amp 12 and the DC/DC converter 15 with a lower efficiency.
FIG. 7(a) is a graph showing one example of variation of the voltage of the node P as a function of time in the follow mode (DC). In FIG. 7(a), a horizontal axis denotes time t and a vertical axis denotes the voltage of the node P.
FIG. 7(b) is a graph showing one example of variation of the voltage of the current detector 14 as a function of time in the follow mode (DC). In FIG. 7(b), the horizontal axis denotes time t and the vertical axis denotes the voltage of the current detector 14.
FIG. 8(a) is a graph showing one example of variation of the voltage of the node P as a function of time in the non-follow mode (DC+AC). In FIG. 8(a), the horizontal axis denotes time t and the vertical axis denotes the voltage of the node P.
FIG. 8(b) is a graph showing one example of variation of the voltage of the current detector 14 as a function of time in the non-follow mode (DC+AC). In FIG. 8(b), the horizontal axis denotes time t and the vertical axis denotes the voltage of the current detector 14.
As shown in FIGS. 7(a) and 7(b), in the case of the DC component in the follow mode, the voltage of the node P is a rectangular wave and a high efficiency switching operation is performed. However, as shown in FIGS. 8(a) and 8(b), in the case of the DC and the AC high frequency in the non-follow mode, a switching operation is performed at a high frequency which is identical to that of the input AC component. Thus, the waveform of the node P is changed from a rectangular wave to a trapezoidal wave, so that the switching loss is large.
As described above, according to the power circuit, a low frequency component is supplied from the DC/DC converter 15 with the high efficiency, and a high frequency component is supplied from the OP amp 12 which can operate at a high speed, so that a high speed operation can be performed with high efficiency.
[First Problem]
FIG. 9 is a graph showing one example of a hierarchical cumulative probability density distribution of a spectrum of an envelope signal in a communication system such as a WiMAX or a LTE. In FIG. 9, the horizontal axis denotes a frequency (MHz) and the vertical axis denotes a hierarchical cumulative probability density distribution (%).
As shown in FIG. 9, in the spectrum of the envelope signal in the communication system such as the WiMAX or the LTE, a component around DC reaches about 90%. In the case of the non-follow mode, the DC/DC converter 15 operates at a switching speed with a low efficiency with respect to the component around DC.
In this regard, the invention is to provide a power circuit which allows a DC/DC converter to operate at an operation speed with high efficiency with respect to the component around DC reaching about 90%, and can achieve high efficiency as a whole.
[Second Problem]
In the power circuit as shown in FIG. 6, the AC component, which can be made to follow due to an increase in the self-oscillating frequency, is increased, that is, the ratio of energy output from the DC/DC converter 15 with the high efficiency is increased, so that the high efficiency of the power circuit can be achieved. However, in the wideband communication system such as the WiMAX or the LTE, since the envelope exists in a wide range, if the switching frequency of the DC/DC converter 15 is raised, switching loss is increased, so that the efficiency of the power circuit is reduced.
Thus, in the wideband communication system, a circuit constant is set such that the AC component of a low frequency is supplied from the DC/DC converter 15 and the AC component of a high frequency is supplied from the OP amp 12. However, even in such a case, the current detector 14 detects the high AC component supplied from the OP amp 12. If the detected AC component exceeds a threshold value of the hysteresis comparator 13, the switch device 32 of the DC/DC converter 15 performs a switching operation, so that loss is increased. Consequently, the efficiency of the power circuit is reduced.
The loss of the switch device 32 of the DC/DC converter 15 will be described with reference to FIGS. 13(a) and 13(b).
FIG. 13(a) is a graph showing one example of a waveform of current I flowing through the switch device 32 as a function of time and a waveform of a voltage V applied between the supply voltage 31 and the node Pas a function of time. In FIG. 13(a), the horizontal axis denotes time t and the vertical axis, denotes amplitude.
FIG. 13(b) is a graph showing one example of the loss of the switch device 32. In FIG. 13(a), the horizontal axis denotes time t and the vertical axis denotes the loss.
As shown in FIGS. 13(a) and 13(b), in the switch device 32, since the voltage V of an off section has a certain value but the current I does not flow, no loss occurs. Since the current I of an on section has a certain value but the voltage V is 0 [V], no loss occurs. The loss of the switch device 32 occurs in a transistion section in which the current I and the voltage V are changed.
Hence, the invention will be described while not considering a loss by an on resistor or the like while focusing on only the switching loss.
Herein, if a switching frequency is increased, that is to say, if the number of transistion sections is increased, the loss is larger. Thus, the relationship between the switching frequency and the efficiency is as shown in FIG. 14.
FIG. 14 is a graph showing one example of characteristics of the efficiency as a function of the switching frequency of the DC/DC converter 15. In FIG. 14, the horizontal axis denotes the switching frequency and the vertical axis denotes efficiency.
Further, even if the switching frequency is constant, the relationship between a duty ratio, which is a ratio of the on section and the off section, and efficiency is as shown in FIG. 15.
FIG. 15 is a graph showing one example of characteristics of the efficiency as a function of the duty ratio of the DC/DC converter 15. In FIG. 15, the horizontal axis denotes the duty ratio and the vertical axis denotes efficiency.
As shown in FIG. 15, if the duty ratio approaches zero, the transistion section approaches and overlaps, so that the loss becomes large. Further, since the switch device 32 does not completely perform a switching operation, the energy of the supply voltage 31 is not converted into output, so that the efficiency is reduced.